The present invention relates to nuclear reactors, particularly liquid metal-cooled fast neutron reactors. In such reactors, the core is mounted within a system of vertically axed vessels ensuring the confinement of the appropriate volume of metal, generally liquid sodium. This volume is surmounted by a neutral gas atmosphere and the system of vessels is closed by a slab, provided with passage orifices permitting access to the core within the vessel. These orifices are closed by rotary plugs. The smallest of the rotary plugs is itself provided with an opening closed by a plug for supporting the means making it possible to inspect and check the reactor core, as well as deflect the hot sodium jet leaving the core. These means and the structure maintaining the same form an assembly called the core cover. The latter is suspended on said plug, inclined in the smallest rotary plug, by a suspension structure and the assembly formed by these three structures, i.e. the plug, the suspension elements and the core cover is called "the core cover plug". The invention more particularly relates to such a core cover plug.
To illustrate the prior art, a description will be given hereinafter with reference to FIG. 1 of a special embodiment of a nuclear reactor core cover plug as described in French Pat. No. 7,429,543 filed on Aug. 29th 1974 by the Commissariat a l'Energie Atomique and published under No. 2,289,031.
In FIG. 1, reference numeral 1 designates a fast neutron nuclear reactor core, which is shown immersed in a liquid metal mass, particularly sodium, which ensures the coring of the reactor. Core 1 is mounted within an inner vessel 2, surrounded by a second, main vessel 3, whose upper part is sealed by a slab 9, which confines the liquid sodium, up to the level indicated at 4, which is surmounted by a covering neutral gas atmosphere 5, generally formed by argon. Vessel 3 is itself surrounded by another vessel, called the safety vessel 6, the system of said vessels with their common vertical axis being arranged within a concrete protection enclosure 7. The latter has a wide opening 8 in its upper part in which is mounted the sealing slab 9. The latter has a central opening 10 for fitting a system with two plugs 11, 12 which, by their mutual rotation, permit access to all points of the core 1. In addition, slab 9 has passages for the fitting of the equipment supported by it, such as pumps and exchangers necessary for the circulation of the sodium. One of each type is diagrammatically shown in FIG. 1, where reference numeral 13 designates a pump and 14 an exchanger. Finally, the core instrumentation is supported by an independent structure, the core cover plug 15, which is itself suspended on the small rotary plug 12.
This core cover plug comprises a head plate 16 level with the small rotary plug 12. This plate supports a cylindrical tube or ferrule 17 which, on its lateral faces, has regularly spaced orifices 18. The ends of intermediate spacers 19, 20 bear on ferrule 17, whose lower part is closed by a thick plate constituting the heat shield 23. Within the ferrule 17 pass vertically the sleeve tubes 26 used for the passage of control rods or instrumentation.
The construction of the hitherto known core cover plugs for liquid metal-cooled reactors suffer from a certain number of disadvantages. In particular, their great rigidity necessary to withstand earthquakes, is obtained by using very thick structures, which gives them a high thermal inertia. This makes it difficult to absorb the very high transient phenomena resulting from variations in the power conditions producing sudden temperature variations in the hot metal leaving the reactor core.